1. Field of the Invention
The present invention relates to an apparatus and a method for lapping a slider, and more particularly relates to an apparatus and a method for lapping a slider which is used in a surface finishing lapping process that is performed after an element height forming lapping process.
2. Description of the Related Art
Sliders used in hard disk drives are fabricated through a wafer process in which read elements and write elements are formed, a process for dicing the wafer into blocks or bars, a lapping process for forming a predetermined air bearing surface, and so on. The lapping process usually consists of two or three separate lapping processes.
First, a rough lapping process, which is may be omitted, is performed in order to improve efficiency in the subsequent element height forming lapping process. In this rough lapping process, a block or a bar having a number of elements that are to be formed into sliders (hereinafter, simply referred to as “elements”) formed thereon is lapped until the read element height reaches a target value thereof. The term “read element height” as used herein means a length (depth) of a read element measured in a direction that is perpendicular to the air bearing surface of an MR (Magneto Resistive) element, and the read element height plays an important role to achieve preferable properties, such as an MR ratio.
Next, in order to accurately form the read element height, a second lapping process called an element height forming lapping process is performed. This lapping process is also called a height adjustment lapping process. Accurate formation of the read element height is significantly important, and a lapping method using resistance elements, such as RLG (Resistance Lapping Guide), is known. The resistance elements are formed in advance between the MR elements in a wafer process, and each resistance element is electrically connected at both ends thereof to pads, which are formed on a surface of a bar that is other than the lapping surface of the bar, via the inside of the elements. During lapping, electric resistance of the resistance elements is measured via the pads. The resistance elements are lapped together with the MR elements, and thereby the electric resistance of the resistance elements is increased as lapping progresses. Thus, it is possible to indirectly estimate the lapping amount of the elements during lapping by obtaining the relationship between an amount in which the elements are lapped and the electric resistance in advance and by lapping the elements while monitoring of the electric resistance of the resistance elements.
However, when a plurality of elements are simultaneously lapped using the above described method, it is not possible to completely prevent variation in the lapping amount among the elements during lapping. Recently, a technology has been disclosed to reduce the variation in the lapping amount during lapping by using a plurality of pressing cylinders for individual elements and thereby applying optimum pressing force to each element (see Japanese Patent Laid-Open Publication No. 2002-157723).
The final lapping process is a so-called surface finishing lapping process, and is often called a touch lap. In the surface finishing lapping process, a mirror finished lapping plate is used to lap the air bearing surface. The surface finishing lapping process removes scratches and the like on the air bearing surface so that smoothness of the air bearing surface can be improved. In this process, a convex shape called a crown is simultaneously formed on the air bearing surface, which is important for flying properties of a slider. In the surface finishing lapping process, the lapping amount itself is not monitored because the lapping amount is small and the pressing force is limited. Lapping is completed when a certain period of time of lapping lapses based on a lapping rate which has been estimated in advance. As means for applying pressing force, Japanese Patent Laid-Open Publication No. 2002-157723 discloses a method for applying optimum pressing force to each element by using a plurality of pressing cylinders, as in the element height forming lapping process. Also, a more simple method is disclosed in Japanese Patent Laid-Open Publication No. 249714/98, in which a weight is put on a lapping head that holds elements.
It is desirable that a lapping plate be as smooth as possible and be accurately mounted with regard to the horizontal direction, but actually the plate is slightly uneven. Therefore, the vertical position of the elements varies relative to the lapping plate during lapping according to the rotation of the lapping plate. If the lapping plate is not accurately mounted with regard to the horizontal direction, the vertical position of elements is also varied during lapping according to the rotation of the lapping plate. Specifically, when the elements pass over a convex portion of the lapping plate, the elements are subjected to increased upward force (thrust) from the lapping plate, leading to an increase in the pressing force against the elements. In a conventional lapping method in which a plurality of elements in a bar are simultaneously lapped, the change in the pressing force may cause variation in the average pressing force among elements in a bar. Moreover, since the surface condition of a lapping plate continuously changes during lapping, the average pressing force also varies among bars. However, it is significantly difficult to keep a lapping plate in a flat and constant condition all the time.
If the average pressing force varies, then elements subjected to large pressing force may be damaged in the worst case. Moreover, since the surface finishing lapping process actually generates a certain amount of lapping, the variation in the element heights that is minimized in the previous element height forming lapping process may be increased again. According to the investigation conducted by the inventors of the present invention, the variation in the element height (MR height) after the surface finishing lapping process is larger than the variation after the element height forming lapping process by about 3 nm. An increase in the recording density of a magnetic head in the future requires a reduction in the element height, and therefore, an increase in the variation in the element height in the surface finishing lapping process makes it difficult to achieve higher recording density of a magnetic head. Variation in the average pressing force may also increase variation in the magnitude of recessions near the read and write element, i.e., PTR (Pole Tip Recession). For example, if a read element is retracted from the air bearing surface relative to the substrate made of Al2O3/TiC, then the distance from a recording medium is increased, and desired reading property is lost. Therefore, large variation in the PRT also leads to a degradation of yield.
Furthermore, if the average pressing force varies, then the lapping plate itself is conversely subjected to large reaction force from the elements at locations of the plate (the concave portions) where large pressing force is applied to the elements. The reaction force may cause fine scratches on the plate, which may reduce the lifetime of the plate because the surface finishing lapping process requires a highly precise mirror finished plate.